A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 2580840 can be evenly divided by 1 2 3 4 5 6 8 9 10 12 15 18 20 24 30 36 40 45 60 67 72 90 107 120 134 180 201 214 268 321 335 360 402 428 535 536 603 642 670 804 856 963 1,005 1,070 1,206 1,284 1,340 1,605 1,608 1,926 2,010 2,140 2,412 2,568 2,680 3,015 3,210 3,852 4,020 4,280 4,815 4,824 6,030 6,420 7,169 7,704 8,040 9,630 12,060 12,840 14,338 19,260 21,507 24,120 28,676 35,845 38,520 43,014 57,352 64,521 71,690 86,028 107,535 129,042 143,380 172,056 215,070 258,084 286,760 322,605 430,140 516,168 645,210 860,280 1,290,420 and 2,580,840, with no remainder.
Since 2,580,840 cannot be divided by just 1 and 2,580,840, it is not a prime number.